Five experiments are proposed. These will determine whether neurons in the brain that project to the spinal nucleus of the bulbo cavernousus muscle (SNB) can accumulate androgen and whether androgen can still exert its morphological effects if these afferents are transsected or if androgen is applied locally in the spinal cord. In Experiment 4, rats that are functionally mosaic for androgen sensitivity will be created to determine whether SNB neurons themselves must be androgen sensitive to undergo morphological alteration. The fifth experiment will examine whether the target muscle affects the efficiency of androgen responsiveness in motoneurons by monitoring testosterone accumulation following axotomy, reinnervation of native muscle, and cross innervation of foreign muscle. In specific aim 2, four experiments will determine how testosterone stimulation of the bulbocavernosus alters SNB dendrites. The role of sensory afferents, axonal transport or neural activity in the motoneuron, and neurotrophic factors will be investigated. In specific aim 3, two additional experiments are proposed: one to examine the role of local NMDA receptors mediate the morphological changes in SNB and a second to examine the role of copulatory experience.